The efficiency of the engine isn't based on it's total power generated, but on other things like the compression ratio and if it's turbocharged or not. A 100hp diesel engine will probably be turbocharged, while its unlikely a 10hp engine would be; turbocharged engines are as a general rule more efficieint than naturally aspirated engines.

The former means that less heat is rejected to radiation and coolant. The latter means that you get more shaft power for a given amount of fuel. Like-for-like, big engines are inherently more thermally efficient.